Conventionally, thermosetting resins such as phenol resin, melamine resin, epoxy resin, unsaturated polyester resin and bismaleimide resin are realized based on thermosetting properties, are excellent in water resistance, chemical resistance, heat resistance, mechanical strength and reliability, and are thus widely used across a wide range of industries.
However, phenol resin and melamine resin are disadvantageous in that volatile byproducts are generated at the time of curing, epoxy resin and unsaturated polyester resin are poor in flame retardancy, and bismaleimide resin is very expensive.
With the goal of solving the above problems, there has been studied polybenzoxazine in which a benzoxazine ring is subjected to a ring-opening polymerization reaction so as to undergo thermal curing without the generation of volatile matter as noted above.
The thermosetting resin having a benzoxazine ring in the molecular structure is a resin in which an oxazine ring is ring-opened by heating and polymerization proceeds without the generation of byproducts, and is thus receiving attention as a thermosetting resin for use in an encapsulating material, impregnation, a laminate, an adhesive, paint, a coating material, a friction material, FRP and a molding material. The benzoxazine ring has a structure that is a combination of a benzene ring and an oxazine ring.
Benzoxazine, having a high glass transition temperature (Tg), low dielectric properties, high tension, a low coefficient of thermal expansion, superb flexibility, and low hygroscopicity, is a curable polymer in which mechanical properties, electrical properties and chemical properties are well balanced.
Techniques for further improving the properties of benzoxazine are being continuously developed. For example, Korean Patent Application Publication No. 10-2012-0058566 discloses a polybenzoxazine composition, in which polybenzoxazine having good thermal stability is prepared by polymerizing a curable composition comprising a benzoxazine compound and a pentafluoroantimonic acid catalyst through heating under conditions of sufficient temperature and time.
Meanwhile, phenol novolac and hexamine have been used as the kinds of resin in conventional rubber compositions. However, the phenol novolac resin suffers from emission of harmful materials such as amine, gas such as formalin, etc. at the time of curing.
Therefore, the present inventors have ascertained that when benzoxazine, having no material generated upon curing compared to resins used for conventional rubber compositions, is utilized as a resin for a rubber composition, superior mechanical properties may be exhibited compared to existing resins, which culminates in the present invention.